It is known to use noble metal chemicals in conjunction with hydrogen gas injection to mitigate intergranular stress corrosion cracking (IGSCC) in nuclear power reactors. As a catalyst, noble metal solution is injected into a reactor to assist in the recombination of oxygen and hydrogen. Delivery of noble metals for power reactors is typically done during hot standby, mode 3. No power is generated during this mode when the noble metal is being injected, resulting in a substantial loss of expensive critical path time.
In addition, during the startup period of a power reactor, hydrogen cannot be injected with the current system configuration. Under the normal water chemistry conditions, an insufficient concentration of hydrogen is available to recombine with radiolytic oxygen. As a consequence, any existing crack will propagate, leading to a portion of the crack that is not treated with noble metal, and hence not mitigated against IGSCC. The on-line injection system of the present invention solves this problem by providing the ability to inject a chemical solution after reactor startup to treat the reactor internals.
Several attempts were made in the chemical delivery process to a power reactor during normal operation. Although each attempt was relatively successful in its outcome, there were major setbacks and improvements with each attempt.
Since main steam line radiation increase is a concern with injecting chemical solutions into the reactor, the injection amount used was very closely monitored with the process controller. The initial injection solution needed to be very dilute to minimize its effect on main steam. With a diluted solution, the storage tank was frequently depleted, requiring multiple labor intensive mixing processes to refill the storage tank. The dilution process was also required every time a concentration change is needed.
To avoid performing the cumbersome solution mixing process, a higher solution concentration was used. However, a higher concentration equates to an increase in the solution's aggressive characteristic, which may have adverse effects on the wetted components of the pump interior. Along with the harsh ambient temperature of 100+° F. in the reactor turbine building, the conventional off-the-shelf injection pumps (several manufacturer tested) failed within hours of operation.
Another concern with the delivery process is loss of chemical due to deposition in the transit line. A shorter residence time in the line would result in less chemical loss. With limited control over volume, boosting the volumetric discharge flowrate with a DI water stream decreased solution residence time in the transit line. This approach also allowed auto-dilution of the chemical, a new feature added to the injection skid.
The on-line injection system of the present invention overcomes the adversities described above to provide uninterrupted delivery of chemical solution into an operating power reactor.